This invention relates generally to projection imaging systems for optical lithography. More particularly, this invention is utilized in an optical lithographic exposure tool to improve the imaging performance through focus of features oriented along x and y axes or directions.
Conventionlal projection imaging tools utilize circular apertures in both the objective lens and condenser lens systems allowing for equivalent imaging of features oriented in any direction. The objective lens has the ability to collect a finite amount of diffracted information from a mask, determined by its maximum acceptance angle or numerical aperture (NA). By limiting high frequency diffraction components, the lens acts as a low pass filter blocking information propagating at angles beyond its capability. Information that is passed is acted on by the lens to produce a second inverse Fourier transform operation, directing a limited reconstruction of the mask object toward the image plane. Depth of focus (DOF) is also a consideration when determining the capabilities of an optical imaging system. Depth of focus is defined as the distance along the optical axis
DOF=+/xe2x88x92k2xcex/NA2, 
that produces an image of some suitable quality. The Rayleigh depth of focus generally takes the form: where k2is a process dependent factor. For a resist material of reasonably high contrast, k2 may be on the order of 0.5. DOF decreases linearly with wavelength and as the square of numerical aperture. Depth of focus is closely related to defocus, the distance along the optical axis from a best focus position. The acceptable level of defocus for a lens system will determine the usable DOF. Tolerable levels of this aberration will ultimately be determined by the entire imaging system as well as the feature sizes of interest.
For the situation described, coherent illumination allows for simplification of optical behavior. Diffraction at a mask is effectively a Fourier transform operation. Part of this diffracted field is collected by the objective lens, where diffraction is in a sense xe2x80x9creversedxe2x80x9d through a second Fourier transform operation.
Partial coherence can be thought of as taking an incoherent sum of coherent images. For every point within a source of finite extent, a coherent Fraunhofer diffraction pattern is produced. For a point source on axis, diffracted information is distributed symmetrically and discretely about the axis. For off axis points, diffraction patterns are shifted off axis and, as all points are considered together, the resulting diffraction pattern becomes a summation of each individual distribution. The zeroth order is centered on axis, but with a width greater than 0, a result of the extent of partially coherent illumination angles. Similarly, each higher diffraction order also has width greater than 0, an effective spreading of discrete orders.
The impact of partial coherence is realized when the influence of an objective lens is considered. By spreading the diffraction orders around their discrete coherent frequencies, operation on the diffracted information by the lens produces a frequency averaging effect of the image and loss of image modulation. This image degradation is not desirable when coherent illumination would allow for superior image reconstruction. If, however, a situation exists where coherent illumination of a given mask pattern does not allow for lens collection of diffraction orders beyond the zeroth order, partially coherent illumination would be preferred.
Consider a coherently illuminated rectangular grating mask where +/xe2x88x92 first diffraction orders fall just outside a projection systems lens NA. With coherent illumination, imaging is not possible as feature sizes fall below the R=0.5 xcex/NA limit. Through use of partial coherent illumination, partial first diffraction order information can be captured by the lens, resulting in imaging capability. Partial coherent illumination, therefor, is desirable as mask features fall below R=0.5 xcex/NA in size. An optimum degree of coherence can be determined for a feature based on its size, the illumination wavelength, and the objective lens NA.
Referring to FIG. 1, a graph of image log slope vs. partial coherence plot for features of size R=0.4 xcex/NA to 0.6 xcex/NA shows how partial coherence impacts imaging through measurement of the aerial image log-slope or ILS. This metric measures the slope of the log of the intensity image produced by the imaging system where larger values correspond to an increase in image fidelity. Changes in this log aerial image gradient directly influence resist profile and process latitude. As this graph illustrates, for increasing levels of partial coherence, image log slope increases for features smaller than 0.5 xcex/NA and decreases for features larger than 0.5 xcex/NA.
For evaluation of DOF using partial coherence, the distribution of diffraction orders needs to be considered. For coherent illumination, there is a discrete difference in optical path length traveled between diffraction orders. By using partial coherence, however there is an averaging effect of the optical path over the lens pupil. By distributing frequency information over a broad portion of the lens pupil, the differences in path lengths experienced between diffraction orders is reduced. In the limit for complete incoherence, the zero and first diffraction orders essentially share a similar pupil area, significantly reducing the effects of defocus. This can be seen in FIG. 2, which is similar to FIG. 1 except a large defocus value has been incorporated. This graph in FIG. 2 shows that at low partial coherence values, ILS is reduced. At high partial coherence levels, however, ILS remains high indicating a greater DOF is possible.
A imaging tool for use with a mask with features oriented along at least an x-axis or a y-axis where the x-axis extends in directions substantially perpendicular to the directions of the y-axis in accordance with one embodiment of the present invention includes an objective lens. The condenser lens has a condenser aperture with four-sides that are located in a condenser lens pupil plane. The sides of the condenser aperture are oriented in substantially the same direction as either the x-axis or the y-axis. The condenser lens is positioned to place at least a portion of illumination on at least a portion of the mask.
A projection imaging system in accordance with another embodiment of the present invention includes a mask, a condenser lens, and an illuminator. The mask has features oriented along at least an x-axis or a y-axis. The x-axis extends in directions substantially perpendicular to the directions of the y-axis. The condenser lens has a condenser plate with a condenser aperture with four-sides which is located in a condenser lens pupil plane. The sides of the condenser aperture are oriented in substantially the same direction as either the x-axis or the y-axis. The illuminator is positioned to illuminate at least a portion of the condenser lens. The condenser lens is positioned to place at least a portion of the illumination on to at least a portion of the mask.
A method for lithography in accordance with another embodiment of the present invention includes a few steps. First, at least a portion of a mask is illuminated onto a condenser lens pupil plane of a condenser lens. The mask has features oriented along at least an x-axis or y-axis with the x-axis extending in directions substantially perpendicular to the directions of the y-axis. The condenser lens has a condenser aperture located in the condenser lens pupil plane. The condenser aperture has four-sides with each of the sides oriented in substantially the same direction as either the x-axis or the y-axis. Next, the illumination passing through the condenser aperture in the condenser lens pupil plane is directed on to at least a portion of the mask.
The present invention provides a number of advantages including improving imaging for lithography, particularly for integrated circuit designs whose feature are primarily oriented in either an X-axis and/or Y-axis directions. The present invention produces noticeable improvements over imaging tools and systems with circular apertures in terms of resolution and focal depth.